1. Field of the Invention
This invention relates to a method and an apparatus for displaying images of a cardiac structure in nuclear examinations. Still more particularly, this invention relates to a method and an apparatus applied to radionuclide angiographic assessment of the heart and particularly the left ventricle. Still more particularly, this invention relates to diagnosis of heart motion by means of a gamma camera.
2. Description of the Prior Art
Radionuclide angiography has been increasingly utilized to evaluate cardiac structures and, in particular, left ventricular function in a wide variety of disease states.
Generally stated, to measure cardiac structure or function with nuclear techniques, the following procedure is followed: A radio-labeled tracer or radionuclide is administered to the patient and distributed in the heart in proportion to the function under investigation. Photons resulting from the decay of the radiotracer are collected by a scintillation camera. This scintillation camera contains a collimator which allows the photons arising only from specific areas of the heart to pass therethrough and to enter a scintillation crystal. In the scintillation crystal the photons interact to cause flashes of light. These light flashes are detected by photomultiplier tubes. Electric signals are transmitted to the scintillation camera electronics where the location of the interactions with the scintillation crystal is determined. Thus, the camera converts the gamma photon energy into an electrical signal that can be processed and displayed. The processed signal is transmitted to a computer system where it is stored and displayed on a screen. The computer is programmed to record the information and permit quantification and optimal display of the data. When the signals from a series of these interactions have been processed and displayed, the information forms a nuclear image.
The design of a specific scintillation camera, namely the Anger-type camera, is disclosed in U.S. Pat. No. 3,011,057, and details of cardiac nuclear imaging are described in "The American Journal of Cardiology", vol. 45, March 1980, pp. 661-673, vol. 46, December 1980, pp. 1109-1115, and vol. 46, December 1980, pp. 1125-1132.
In radionuclide cineangiography, a Fourier analysis technique was suggested in order to obtain images from acquired cardiac data (Geffers H., Adams W. E., Bitter F., et al.: Data Processing and Functional Imaging in Radionuclide Ventriculography; Proceedings of Fifth International Conference on Review of Information Processing in Medical Imaging, Nashville, TN, 1977).
In this previous approach the time activity curves (counts plotted versus time) for all single pixels are subject to a Fourier analysis. In this Fourier analysis, a Fast Fourier Transform (FFT) computer program is applied to each time-activity curve and used to evaluate or extract the amplitude A and phase .phi. of the fundamental frequency of the heart cycle. The amplitudes A and the phases .phi. are imaged for all pixels. Yet, there are some shortcomings. Fourier analysis techniques require (a) data which span a full cycle of the fundamental frequency, and the practical application via the FFT computer program requires (b) the number of sampled points to be a power of two. The first requirement (a) is difficult to achieve when there is a data drop-out in the time-activity curves due to variations of the R--R interval. To insure the second requirement (b), extra data manipulation is usually needed. A third shortcoming of the Fourier technique is that the fundamental period is not easily varied to match the "natural" heart period. Often, there is a period of diastis in the volume curve which causes the average R--R interval and the "natural" heart period to differ significantly. The fundamental period in the Fourier case is, by necessity, the R--R interval.
Even though the parameters A and .phi. are valuable tools in evaluating the amount of heart motion and the point of time when such motion occurs, there is no indication for the observing physician how reliable these parameters A and .phi. are.